1. Field of the Invention (Technical Field)
The present invention relates generally to internal combustion engine (ICE) exhaust noise mufflers, specifically a dissipative muffler with improved maintenance, noise attenuation, durability features and reduced impact on engine efficiency.
2. Background Art
Prior art shows dissipative mufflers, which are commonly composed of an inlet port fluidically connected to an outlet port by a duct that also forms the inner wall of an annular chamber containing acoustically absorptive fill. Currently, dissipative mufflers often use a perforated metal liner defining a duct that provides a boundary between the flow of gas and the surrounding volume of acoustically absorbent fill. In typical mufflers, the absorbent fill initially is contained between the inner duct and an outer casing. In some mufflers, a perforated metal duct serves as a backing or facing for a liner made from another material, e.g., fiberglass cloth.
Some muffler apparatuses known in the art include those disclosed in the following U.S. Pat. Nos.:
Also, U.S. Pat. No. 5,162,620 to Ross provides particularly helpful background to the present invention.
According to Schultz, perforated metal has a xe2x80x9cself flow resistancexe2x80x9d (Schultz, Acoustical Uses For Perforated Metals, p. 56) and a xe2x80x9ctransparency indexxe2x80x9d (Schultz, p. 14) which can be calculated from the following:
Self Flow Resistance=Rself=R0+2xcex94R0; where R0 is the greater of
where R01=4.24(b2t/d3)f0.5
and R02=2.88(b2t/d4)
and 2xcex94R0=4.19(b2/d2)f0.5xc3x9710xe2x88x923 cgs rayls.
Also,
Transparency Index=TI=(nd2/ta2)=0.04P/(3.14ta2)
With the above variables defined as follows:
a=shortest distance between holes (a=bxe2x88x92d)
b=on-center hole spacing
d=perforation diameter
f=frequency
n=number of perforations per unit area
P=percentage open area
t=thickness of sheet
Thus, muffler ducts fashioned from ordinary perforated metal are considered reasonably xe2x80x9ctransparentxe2x80x9d to sound; but, due to their modest flow resistance, they also permit diversion of conveyed gas flow into the chamber containing the acoustically absorbent media. Not only does this diversion create turbulence and static pressure loss, it can actually entrain or xe2x80x9cblow outxe2x80x9d fill media through the perforations and through unsealed muffler casing-to-endcap connections. This xe2x80x9cblow outxe2x80x9d problem is commonly encountered and well-known by users of conventional dissipative mufflers.
Ingard, (Sound Absorption Technology, 1994, p. 4-25) shows the normalized flow resistance of most perforated metals, i.e., the ratio of the flow resistance of the perforated metal sheet over the acoustical impedance of the gas flow, is near zero for most internal combustion (ICE) muffler applications and thus, when studied in combination with the fill it is lining, is excellent for preserving virtually ideal acoustical absorption at mid to high frequencies. However, effective absorption coefficient drops dramatically in the low frequency end of the overall spectrum, with absorption worsening with increasing wavelength. The resulting poor low frequency attenuation plagues all dissipative prior art designs utilizing perforated metal as a fill liner.
Thus, for ICE and other gas flow applications that have significant low frequency sound characteristics, reactive-type mufflers incorporating single or multiple chambers and tuned Helmholz resonators are usually preferred over dissipative muffler designs when low frequency noise reduction is a primary objective. Reactive mufflers, because they do not contain acoustically absorptive fill in their design, are also perceived as offering xe2x80x9cconsistentxe2x80x9d performancexe2x80x94i.e., they don""t degrade or xe2x80x9cblow out,xe2x80x9d and require frequent replacement or re-packing of dissipative media like fiberglass fill. In today""s marketplace, dissipative mufflers are usually regarded as xe2x80x9crace pipesxe2x80x9d that have far less backpressure than tortuous path reactive muffler designs, and thus have a reduced adverse impact upon engine horsepower, but at the expense of less low frequency noise reduction. In many instances, these xe2x80x9cglass-packsxe2x80x9d are desired for that purpose, and are installed to preserve deep and powerful-sounding low frequency engine exhaust tones.
When broad-band acoustic attenuation is required, a muffler can feature both reactive and dissipative elements either in series or parallel, with performance anticipated much in the same way one would design an electrical circuit. Such mufflers, however, can become quite complicated and heavy, as certain portions contain fill, while other portions have solid partitions. Additionally, due to the reliance on reactive methods for low frequency attenuation, even the combination muffler designs suffer high pressure losses and reduce the engine""s overall performance.
Another sound attenuation technique known in the art, primarily for aerospace and industrial applications, is the use of components crafted from fibrous sintered metal (a.k.a. fiber metal) as a high flow resistance facing for empty cavities that resemble Helmholz resonators. The understood purpose of the cavity is to provide, like a Helmholz resonator, a quarter-wavelength distance which enables the facing material to intercept specific waveforms at their maximum amplitudes and thus yield highest attenuation for a narrow band of frequencies. The published literature (Clark, xe2x80x9cTurning Down the Volumexe2x80x9d, Machine Design, Sep. 24, 1993) summarizes the function of the fiber metal as an alternative form of dissipative attenuation which can replace traditional fill. Sales collateral from one manufacturer of fiber metal carries this theme further by noting disadvantages of fiberglass media when compared to the fiber metal faced cavity attenuation technique. Nowhere is suggestion made, however, that the cavities might be occupied with acoustically absorbent fill, or that the fiber metal element serves only as a liner or container for another material.
Two of Clark""s U.S. Pat. Nos., 3,955,643 and 3,920,095, reiterate the use of fiber metal as a facing for empty Helmholz-like cavities. In the former, fiber metal is used in conjunction with other flow-resistive materials to furnish a cavity liner with xe2x80x9ccontinually increasingxe2x80x9d flow resistance. In the latter, fiber metal faced cavities are part of a combination muffler device designed to produce low and high frequency attenuation.
Yet another technique for improving sound attenuation in a muffler is to use linear occlusion of the gas flow path. In such a technique, what would otherwise be a clear line-of-sight between the inlet and outlet ports of a muffling device is blocked or obscured by obstructions, offsets, turns, or some other means. Prior art shows many ways linear occlusion can be provided, as exhibited by the following reference list of U.S. Pat. Nos.:
But while such means for linear occlusion may provide desirable improvements in sound reduction, there is usually a dramatic performance cost manifested by increased backpressure in the muffler. Therefore, it may be desirable to implement the least flow resistive means of linear occlusion while gaining as much noise attenuation as possible. For example, as some of the above references disclose, helical or spiral flow passages avoid the use of highly restrictive ninety-degree or reverse-turning elbows, yet still provide linear occlusion. A study of the prior art featuring such flow passage geometries resulted in the following findings: Itani (U.S. Pat. No. 4,635,753) suggests a dissipative muffler design with coaxial spiraling polygonal ducts. Taniguchi (U.S. Pat. No. 4,303,143) demonstrates spiraling blades. Fisher (U.S. Pat. No. 1,341,976) utilizes a solid-looking helical member, with or without varying pitch, inside a close-fitting casing. Flint (U.S. Pat. No. 2,482,754) also uses a solid helical twist of sheet metal, and specifies the length must be ten times the diameter. Smith (U.S. Pat. No. 3,235,003) calls for spiral plates that may be solid or perforated. DeVane (U.S. Pat. No. 3,696,883) describes a helical-shaped baffle assembly which makes use of bars and spokes for internal support and attachment to the surrounding flow duct. De Cardenas (U.S. Pat. No. 3,746,126) suggests a flat bar twisted into a helix, with pitch equal to half the diameter. DeVane (U.S. Pat. No. 4,667,770) requires a tubular frame and other parts comprising yet another helical embodiment of linear occlusion. Kojima (U.S. Pat. No. 4,533,015) shows a plurality of helical members arranged sequentially inside a flow duct. Bokor (U.S. Pat. No. 6,089,348) makes use of a spiral vane in the reactive section of a series combination muffler design. Johnston (U.S. Pat. No. 6,167,699) incorporates half-twist helical strips inside specific pipe sections of a larger assembly. Calciolari (U.S. Pat. No. 5,443,371) utilizes a helical insert to help reduce compressor noise.
While the prior art perhaps suggests the function of, for instance, a linearly occluding helical insert in its capacity to scatter, deflect, or otherwise affect sound waves traversing the muffler duct, to the inventor""s knowledge nothing in the known art calls for use of an impedance-matching material as a means of linear occlusion.
The invention is an apparatus and method for improved sound attenuation in mufflers, especially mufflers for internal combustion engines. The use of fiber metal or similarly high flow resistance and high acoustic transparency material as a liner for traditional acoustically absorptive media in a dissipative muffler exhibits improved low frequency sound attenuation, reduces backpressure, and eliminates media entrainment or xe2x80x9cblow-outxe2x80x9d phenomenon which results in longer muffler life. The same class of materials may also be used to fashion an element that provides linear occlusion inside an otherwise line-of-sight type of muffler, where the occluding element provides improved impedance-matching acoustic absorption. Disclosed embodiments providing linear occlusion minimize traditional increases in muffler backpressure by incorporating helical, conical, and annular members in mufflers with round ducts. To maximize attenuation, a muffler according to the invention may feature both a fiber metal fill liner and a fiber metal linear occlusion element. Further, the liner that connects the inlet and outlet ports of the muffler may feature an offset, elbow, or turn that would simultaneously allow it to provide means for linear occlusion.
There is provided according to the invention a sound attenuating apparatus for conveying internal combustion engine exhaust gases, the gases having an acoustical impedance, the apparatus comprising an inlet port and an outlet port, a rigid duct fluidically connecting said ports, said duct having a flow resistance and defining an inner wall of a chamber, and means for acoustic absorption disposed in said chamber, wherein said duct has a transparency index greater than 100,000 as calculated from Schultz""s formula, and further wherein the ratio of the flow resistance of said duct to the acoustic impedance of said exhaust gases is between approximately 0.2 and approximately 2.0. The duct may be composed of a single material or a plurality of materials. In a preferred embodiment of the invention the duct provides linear occlusion between said ports.
There is also provided a sound attenuating apparatus for conveying internal combustion engine exhaust gases, the gases having an acoustic impedance, the apparatus comprising an inlet port and an outlet port fluidically connected by a rigid duct, said duct defining an inner wall of a chamber filled with means for acoustic absorption, and means for linear occlusion disposed within said duct, said linear occlusion means having a transparency index greater than about 100,000 as calculated from Schultz""s formula, and said linear occlusion means also having a flow resistance, wherein the ratio of the flow resistance of said linear occlusion to the acoustic impedance of said exhaust gases results is between 0.2 and 2.0. Preferably but optionally, the means for linear occlusion is removable from within said duct.
A sound attenuating apparatus for conveying internal combustion engine exhaust gases according to the invention may also comprise an inlet port and an outlet port fluidically connected by a rigid duct, said duct having a transparency index greater than 100,000 as calculated from Schultz""s formula, and also a flow resistance; and a chamber, substantially filled with means for acoustical absorption and having an inner wall defined by said duct, wherein the ratio of the flow resistance of said rigid duct over the acoustic impedance of said exhaust gases results is between 0.2 and 2.0; and means for linear occlusion disposed within said duct, said linear occlusion means having a transparency index greater than 100,000 as calculated from Schultz""s formula and also a flow resistance; wherein the ratio of the flow resistance of said linear occlusion over the acoustic impedance of said exhaust gases is between 0.2 and 2.0. In one embodiment the means for linear occlusion comprises a helical member, which optionally is removable from within said duct. In the preferred embodiment of the invention, the means for linear occlusion comprises metal fiber. In the preferred embodiment of the invention, the duct also comprises metal fiber, and optionally but preferably provides linear occlusion between said inlet and outlet ports.
In one particular embodiment of the invention, a muffler has an inlet port and an outlet port fluidically connected by a rigid duct, said duct defining an inner wall of a chamber filled with means for acoustic absorption; and a helical member disposed within said duct, said member having a transparency index greater than about 100,000 as calculated from Schultz""s formula, and said helical member also having a flow resistance; wherein the ratio of the flow resistance of said helical member to the acoustic impedance of said exhaust gases results is between approximately 0.2 and approximately 2.0.
A further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.